1. Technical Field
This disclosure relates to semiconductor fabrication and more particularly, to a method for forming a high quality silicon oxide which is uniform in thickness on more than one crystallographic silicon surface.
2. Description of the Related Art
Three-dimensional (3D) integrated circuits offer an extremely high density of electronic devices per unit of surface area. Various 3D memory cells include vertical field effect transistors with their gates placed on the vertical wall of a via or a trench. Because the vertical walls of an opening may have more than one crystallographic orientation of Si crystal, the thin gate dielectric film has to be formed on silicon surfaces with different crystallographic orientations for the silicon.
The gate insulator layer has to be substantially uniform and have a high quality Si/insulator interface for proper operation of a transistor.
Silicon dioxide (SiO2) or silicon oxynitride (SioxNy) grown by thermal oxidation of Si in different oxygen-bearing ambients are known to have a high quality Si interface; however, the rate of silicon oxidation is different for Si planes with different crystal orientation. (see, for example, FIGS. 1-4).
Referring to FIG. 1, an oxidation rate of silicon oxide films are shown as a ratio of the oxidation rate in the (100) crystal plane and the oxidation rate in the (110) crystal plane versus temperature. The data displayed was published by E. A. Irene, H. Z. Massoud, and E. Tierny in J. Electrochem. Soc. 133, 1253 (1986).
Referring to FIG. 2, a dry rapid thermal oxidation (RTO) process is graphically shown in a plot of oxidation thickness versus time for the (100) crystal plane and the (110) crystal plane of silicon. Referring to FIG. 3, a dry oxidation process is plotted for oxidation thickness versus time for the (100) crystal plane and the (110) crystal plane of silicon. FIG. 4 shows a wet (water vapor furnace) oxidation process plotted for oxidation thickness versus time for the (100) crystal plane and the (110) crystal plane of silicon.
FIGS. 1-4 show the disparity in thickness and oxidation rates for oxides on silicon with respect to the different crystallographic planes of silicon. It is, therefore, a challenge to grow a high-quality uniform film on the Si surface with more than one crystallographic orientation.
Referring to FIG. 5, a cross-sectional view of a trench 10 formed vertically (e.g., extending into the plane of the page) in a silicon substrate 12 is shown. The non-uniformity of a thin oxide film 14 grown thermally by a wet furnace method at 800 degrees C. for 8 minutes is shown. Oxide film 14 is grown on the sidewall surfaces of the vertical trench 10. The interior of the trench includes surfaces with the different crystallographic orientations of Si, namely (100) and (110). Consequently, the oxide film thickness varies along the trench perimeter depending on the crystal orientation. For example, the oxide film 14 is thicker at locations a, c, e and g corresponding to plane (110), and thinner at locations b, d, f and h corresponding to plane (100) as shown.
FIGS. 1-5 show that the oxidation rates depend strongly on the crystallographic orientation for standard oxidation processes. Rapid thermal processing (RTP) and furnace oxidation in both pure oxygen and water vapor result in a difference between the oxidation rates of (100) and (110) planes of between 40% and 100% depending on the ambient, processing temperature, and film thickness. There is a fundamental reason that explains this behavior. Si atoms have different packing density on different crystallographic planes. Generally, silicon oxide films grow faster on Si surfaces with higher atomic packing density.
Therefore, a need exists for growing an oxide, on silicon, of uniform thickness despite the presence of more than one crystallographic plane.
A method for forming an oxide of substantially uniform thickness on at least two crystallographic planes of silicon, in accordance with the present invention, includes providing a substrate including surfaces having at least two different crystallographic orientations of the silicon crystal. Atomic oxygen (O) is formed for oxidizing the surfaces. An oxide is formed on the surfaces by reacting the atomic oxygen with the surfaces to simultaneously form a substantially uniform thickness of the oxide on the at least two different crystallographic orientations of the surfaces.
Another method for forming an oxide of substantially uniform thickness on at least two crystallographic planes of silicon, in accordance with the present invention, includes the steps of providing a substrate, forming a continuous surface including at least two different crystallographic orientations of the silicon crystal on the substrate, forming atomic oxygen to react with the continuous surface, and forming an oxide on the continuous surface by reacting the atomic oxygen with the continuous surface to simultaneously form a substantially uniform thickness of the oxide on the at least two different crystallographic orientations.
In other methods, the step of forming atomic oxygen for oxidizing the surfaces may include the step of forming atomic oxygen by employing a free radical enhanced rapid thermal oxidation (FRE RTO) process. The step of forming atomic oxygen for oxidizing the surfaces may include the step of forming atomic oxygen by employing remote electrical discharge of oxygen. The step of forming atomic oxygen for oxidizing the surfaces may include the step of forming atomic oxygen by decomposing an unstable oxygen-bearing gas. The unstable oxygen-bearing gas may include ozone. The at least two different crystallographic orientations may include two of planes (100), (110) and (111). A maximum thickness difference of the oxide film grown on the at least two different crystallographic planes is less than 20%. The step of providing a substrate including surfaces having at least two different crystallographic orientations of the silicon crystal may include the step of forming a three dimensional structure in the substrate. The three-dimensional structure may include a trench, a pillar or a via. The step of annealing the oxide may be included. The step of annealing the oxide may include the step of annealing the oxide at a temperature between 600 degrees C. and 1200 degrees C. in an ambient. The ambient may include at least one of: O2, N2, H2, N2O, NO, NH3, and Ar.
These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.